No, it was introduced with WoD.
During most of MoP it was: LFR, Norm, Heroic.
During SoO (last raid of MoP) they added flex and it became: LFR, Flex, Norm, Heroic.
Only Flex was flexible.
During WoD they changed it to: LFR, Norm, Heroic, Mythic
Norm and heroic are now flexible.
As for the armory, it’s very weird about those raids… I don’t really trust it. For some of my characters it says I have no kills in those raids, when I know that I did.